1. Field of the Invention
The present invention relates to a touch probe which carries a surface contacting stylus, and is used on a coordinate positioning machine (such as a coordinate measuring machine or machine tool) to sense the position of, for example, a workpiece surface. More particularly, the invention relates to a touch probe on which a number of different styli may be interchangeably supported.
One type of known touch probe is a "touch-trigger" probe. Touch-trigger probes emit a trigger signal upon sensing contact between a stylus carried by the probe and a surface. Typically, such a probe comprises a fixed structure, such as a housing, by which the probe may be supported on the movable arm of a coordinate positioning machine, and a stylus-supporting member, to which the stylus is connectable, supported within the housing in a rest position. The stylus-supporting member may be displaced from the rest position when a deflecting force is applied to the stylus, typically as a result of contact between the sensing tip of the stylus and the surface of a part to be inspected, during movement of the movable arm, and thus the probe, relative to the surface (this is known as "overtravel"). A biasing force is provided to bias the supporting member into the rest position, so that irrespective of the orientation of the probe, upon removal of the deflecting force the supporting member returns precisely to the rest position. Such a probe is known from U.S. Pat. No. 4,153,998.
It is frequently necessary to inspect parts of a relatively complex shape, having surfaces which are oriented in a variety of directions. One way to perform such inspection is to provide a probe with a suitably shaped stylus, which has one or more sensing tips oriented in such a way that the surface of the part to be inspected my easily be contacted. In order to provide a truly flexible measuring system however, it is desirable to provide automatic changing of one or more such styli on a probe.
2. Description of Related Art
Such an automatic stylus-changing system is known from, for example, U.S. Pat. No. 4,637,119, in which an electromagnetic clamping device is provided on the stylus-supporting member to enable releasable clamping of a plurality of different styli to the stylus-supporting member.
A further type of stylus changing system is known from U.S. Pat. No. 4,349,946 and EP 343237, in which styli are releasably retained by means of a bayonet-type connection. In U.S. Pat. No. 4,349,946 the bayonet connection is provided between the stylus and stylus-supporting member, with the relative rotation required to perform an exchange operation being provided by a motor in the probe which rotates the stylus-supporting member. EP 343237 discloses a releasable bayonet connection between an entire stylus-supporting member and probe; the relative rotation for an exchange operation is generated by means of relative translational motion between the probe and a magazine containing the stylus-supporting member.
All of the above-mentioned stylus-changing systems are relatively complex. Additionally, each of these systems suffers from the problem that different lengths of styli ideally require different biasing forces (a) due to the different weights of their differing lengths and (b) to prevent overloading of short styli. Manual adjustment of the biasing force each time a stylus is changed is impractical since such a stylus changing system is ideally automated to provide flexible part inspection. Automatic adjustment mechanism for the biasing force (such as the one shown in U.S. Pat No. 5,024,003) are complex and undesirably add to the weight of the probe.